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Fibre Reinforced Polymers

Fibre Reinforced Polymers (FRP) are composite materials comprising a polymer matrix reinforced with fibres. The fibers are usually glass, carbon, or aramid fibres, while the polymer is usually an epoxy, vinylester or polyester thermosetting plastic. FRPs are commonly used in the aerospace, automotive, marine, and construction industries.

The primary role of the matrix in composites is to provide lateral support and protection to the fibres and the choice of the type of matrix is dictated by compatibility with the FRP manufacturing process, mechanical properties of the fibres and other chemical attributes. Due to the non-structural importance of the resins, as well as their high cost, a minimum resin volume ratio is always desirable. However, the maximum fibre ratio that can be achieved is normally below 70%.

FRP materials can be manufactured by using different techniques such as pultrusion, filament winding, moulding, braiding and manual lay-up and can be produced in various shapes. As for conventional steel reinforcement, non-ferrous composite materials are manufactured in forms of rebars, sheets, grids and links. The most common method of producing FRP rebars is pultrusion, in which the fibres are continuous and unidirectional. Several shapes can be made but the most common shape used is circular. Links, however, are more difficult to produce. Currently available links are made by post-curing of fibre bundles pulled wet or by filament winding. Thermosetting resins have also been used, since they allow bending of FRPs after manufacture. Composites can be engineered to meet the specific demands of each particular application and their overall performance and characteristics depend on the choice of materials (fibre and matrix), the volume fraction of fibre and matrix, fibre orientation, fabrication method. Furthermore, in order to enhance the bond characteristics of FRP reinforcing bar in concrete, several techniques are used including surface deformations, sand coating, over-moulding a new surface on the bar or a combination of processes. Figure 1 shows the generic mechanical properties of FRP reinforcement according to the type of fibres used in their manufacture.

Figure 1: Tensile properties for steel and a range of FRP reinforcements

FRP products are characterized by a perfectly elastic behaviour up to failure and can develop higher tensile strength than conventional steel in the direction of the fibres. This anisotropy, however, seriously affects the shear strength, which is very low, compared to the tensile strength, and depends on the properties of the matrix and orientation of the fibres. It also reduces the ability of the composite to resist forces perpendicular to the direction of the fibres. The elastic modulus of FRP materials used in construction generally varies between 20% of that of steel for glass fibres to 75% of that of steel for carbon fibres. As a result, more flexible RC elements are obtained which develop higher strains in tension and reach higher overall deformations. Higher strains in tension and smaller areas in compression are expected to influence the shear behaviour. Although these materials, in general, have a low compressive strength, due to low buckling strength of the individual fibres, this is not usually a concern since, in the majority of civil engineering applications, these elements are essentially used only in tension. The main advantages and disadvantages of these advanced composite materials versus steel are listed in Table 1.

Table 1: Advantages and disadvantages of FRP reinforcement
Advantages Disadvantages
higher ratio of strength to self weight (10 to 15 times greater than steel ) higher raw material cost
carbon and aramid fibre reinforcement have excellent fatigue characteristics lower elastic modules (except some Carbon FRPs)
excellent corrosion resistance and electromagnetic neutrality Glass FRP reinforcement suffers from stress corrosion
low axial coefficient of thermal expansion lack of ductility

High strength and low elastic modulus compared to steel are the distinctive properties of FRP materials. As a result of this, and the fact that FRP materials, unlike steel, do not offer plasticity, a different behaviour is expected for FRP compared to steel RC elements. This leads not only to different load-deflection characteristics, but also to a change in the mode of failure, which becomes substantially brittle, even in flexure, and consequently, can cause major problems when designing according to the same philosophy as was developed for steel RC structures.

Chronological developments in the field